1. Field of the Invention
The present invention relates to an electronic device, in particularly to a protective device capable of protecting electronic apparatus having it from damage by excessive current or excessive voltage.
2. Description of Related Art
In order to protect battery and battery charger from damage caused by excessive current or excessive voltage while charging is performed, a protective device is often put into the battery charger. Thus, when the excessive current or voltage is applied on the battery charger, the protective device can interrupt the circuit therein immediately and protect the battery and the electronic components in the battery charger.
FIG. 1 is a circuit diagram of a battery charger. There is a protective device 11 in the excessive current/voltage protective circuit 10 of the battery charger. The protective device 11 has two current fuses 111 and 112 arranged between the node A and node B. The current fuses 111 and 112 are made of low melting point metal and can be broken by excessive charging current passing therethrough. Consequently, the circuit between the node A and node B are interrupted and the battery 12 and the electronic elements in the battery charger can be protected.
Besides, the excessive current/voltage protective circuit 10 has an integrated circuit 13 for detecting excessive voltage. Once an excessive voltage is detected, the integrated circuit 13 will conduct a MOSFET 14 and the electrical current thus can be allowed to pass through path C. Then the heating member 113 of the protective device 11 generates heat for melting the current fuses 111 and 112 and a breakage is formed for protecting the battery 12 and the battery charger.
More specifically, as FIG. 2 shows, the protective device 20 has a substrate 21, two first electrodes 22 respectively formed at two opposite sides of the substrate 21, and a low melting point metal layer 23 electrically connected across the two first electrodes 22. A current path is formed from one of the first electrodes 22 to the low melting point metal layer 23 and then to the other one of the first electrodes 22. So once excessive current enters either of the first electrodes 22, the low melting point metal layer 23 will melt to break and form a breakage between the two first electrodes 22.
As shown in FIG. 2 and FIG. 3, the protective device 20 has two second electrodes 24 formed at another two opposite sides of the substrate 21. The two second electrodes 24 each have an extending portion 241 extending under the low melting point metal layer 23. A heating member 25 is formed between the two extending portions 241. An insulating layer 27 is provided for covering the heating member 25 and the second electrodes 24. Another current path is formed from one of the second electrodes 24 to the heating member 25 and then to the other one of the second electrodes 24. Once current with excessive voltage enters either of the second electrodes 24 of this current path, the heating member 25 will generate heat for melting and breaking the low melting point metal 23 and form a breakage. In addition, the second electrode 24 at the right side of FIG. 3 has a third electrode 242 and electrically extending to the low melting point metal layer 23.
In order to rapidly break up the low melting point metal layer 23, an appropriate amount of flux 26 is applied on the low melting point metal layer 23 for preventing oxidation occurred on the surface of the low melting point metal layer 23. Besides, the flux 26 can remove the oxide layer formed on the low melting point metal layer 23 and help to increase the breaking thereof. The main composition of the flux 26 is rosin, which has a liquidus temperature as low as between 50 to 80 degrees Celsius. When the protective device 20 is being connected to a circuit board in a reflow soldering process, the high temperature over 200 degrees Celsius therein will immediately evaporate the flux or drive it to move away. Without the flux, the low melting point metal layer 23 will not easily be melted to break when an excessive current or voltage is applied on the protective device 20, and the protective device 20 will fail to give any protection to the battery charger or the battery.